The invention relates generally to implantable devices characterized by gradients of materials, architecture, and/or properties for tissue regeneration, made using solid free-form fabrication technology, which can be combined with computer-aided design.
Bone deficit or defects may result from congenital defects, disease, aging, or trauma. Bones are composed of highly vascularized tissue, called osseous tissue, which harbors blood-forming elements, the marrow. The external and internal structure of bone is in dynamic flux: the cellular elements produce and remodel a matrix of cartilage in which calcium salts are deposited. Approximately two-thirds of mature bone is calcium phosphate as hydroxyapatite, one third is predominantly collagen fibers and other calcium salts, while only 2% by weight is living cells. Through processes of calcium resorption and deposition, osteocytes and various other cell types are able to remodel or heal bone as needed while the skeleton continues to provide structural support for the body.
Bone is further divided into dense (compact) and spongy (cancellous) areas. Because it has the greater mechanical strength, compact bone is positioned to receive the greatest physical loads generated by the weight of the body and skeletal muscle contractions. Compact bone forms along the surface of the long axis of the long bones forming what is known as the cortex of the bone. Spongy bone composes the xe2x80x9cheadxe2x80x9d (epiphysis) and the inner areas of the bones, and borders the medullary cavity in the larger bones.
Cartilage, on the other hand, is an avascular tissue composed of 5-10% by weight of living cells. There are three major types of cartilage in the body: hyaline, fibrocartilage, and elastic cartilage. Hyaline cartilage covers the epiphyses of the bone and, in synovial joints, lies within a fluid filled capsule. Fibrocartilage composes the intervertebral discs separating the vertebrae of the spinal columns. Elastic cartilage is present in areas requiring extreme resilience, such as the tip of the nose. Cartilage is formed by and contains cells called chondrocytes. The extracellular matrix of hyaline cartilage contains closely packed Type II collagen fibers and proteoglycans including hyaluronate and glycoaminoglycans in a chondroitin sulfate matrix. Chondrocytes receive nutrients and dispose of wastes by diffusion through the matrix and are believed to have limited mobility or ability to divide and regenerate damaged tissue. Chondrocytes normally produce anti-angiogenesis factors. However, when large areas of cartilage are damaged, overgrowth by fibroblasts and neovascularization of the area may result in the formation of scar tissue or callus instead of articular cartilage. A subsequent ingrowth of bone forming cells may result in calcium deposition in these areas, causing further deformation of the local area.
The interface between bone and cartilage is therefore the interface between a vascularized and avascular tissue as well as mineralized (ossified) and nonminerilized collagen matrices. Traumatic injury, as well as such conditions as osteoarthritis and aging, often result in damage to the articular cartilage, which may also involve damage to the underlying bone. Therefore, there is a need for a method of treatment which meets the disparate needs of both tissue types and allows or encourages the healing process to progress towards restoration of both types of tissues at the same site.
Clinical use of grafts of living tissue have recently moved from direct implantation of freshly harvested fully formed tissue, e.g. skin grafts or organ transplants, to strategies involving seeding of cells on matrices which will regenerate or encourage the regeneration of local structures. For complex and weight bearing hard tissues, there is an additional need to provide mechanical support of the existing structure by replacement or substitution of the hard tissue for at least some of the healing period. Thus, the device must serve as a scaffold of specific architecture which will encourage the migration, residence and proliferation of specific cell types as well as provide mechanical and structural support during healing. In the case of devices for regeneration of articular (hyaline) cartilage, it is important that the device be completely resorbable, as residual material may compromise the surface integrity (smoothness) and overall strength and resilience of the regenerated tissue.
In order to encourage cellular attachment and growth, the overall porosity of the device is important. Additionally, the individual pore diameter or size is an important factor in determining the ability of cells to migrate into, colonize, and differentiate while in the device (Martin, R B et al. Biomaterials, 14: 341, 1993). For skeletal tissues, bone and cartilage, guided support to reproduce the correct geometry and shape of the tissue is thought to be important. It is generally agreed that pore sizes of above 150 xcexcm and preferably larger (Hulbert, et al., 1970; Klawitter, J. J, 1970; Piecuch, 1982; and Dennis, et al., 1992) and porosity greater than 50% are necessary for cell invasion of the carrier by bone forming cells. It has been further accepted that a tissue regenerating scaffold must be highly porous, greater than 50% and more preferably more than 90%, in order to facilitate cartilage formation.
It is well documented that the physiological processes of wound healing and tissue regeneration proceed sequentially with multiple cell types and that cellular factors play a role. For example, thrombi are formed and removed by blood elements, which are components of cascades regulating both coagulation and clot lysis. Cells which are not terminally differentiated, such as fibroblasts, migrate into the thrombus and lay down collagen fibers. Angiogenic cells are recruited by chemotactic factors, derived from circulating precursors or released from cells, to form vascular tissue. Finally, cells differentiate to form specialized tissue. The concept of adding exogenous natural or synthetic factors in order to hasten the healing process is also an area of intense exploration, and numerous growth factors, such as cytokines, angiogenic factors, and transforming factors, have been isolated, purified, sequenced, and cloned. Determining the correct sequence and concentration in which to release one or multiple factors is another area of research in the field of tissue engineering.
Several attempts to address some of the above problems of tissue regeneration in a graft or implantable device have been disclosed. U.S. Pat. No. 5,270,300 describes a method for treating defects or lesions in cartilage or bone which provides a matrix, possibly composed of collagen, with pores large enough to allow cell population, and which further contains growth factors or other factors (e.g. angiogenesis factors) appropriate for the type of tissue desired to be regenerated. U.S. Pat. No. 5,270,300 specifically teaches the use of TGF-beta in the matrix solution as a proliferation and chemotactic agent at a lower concentration and at a subsequent release of the same factor at a higher concentration to induce differentiation of cartilage repair cells. In the case of a defect in adjoining bone and cartilage, a membrane is secured between the bone-regenerating matrix and the cartilage-regenerating matrix to prevent blood vessel penetration from one site to the other site.
U.S. Pat. No. 5,607,474 to Athanasiou et al. describes a molded carrier device comprising two bioerodible polymeric materials having dissimilar mechanical properties arranged proximate to each other for the purpose of being placed in the body adjoining two dissimilar types of tissues. Each polymeric material has a variable degree of porosity or pore sizes into which tissue cells can enter and adhere. The two components of the device are fabricated separately and, e.g., bonded together in a mold. Other features, such as larger passages for cell access, can be mechanically placed in the device.
U.S. Pat. No. 5,514,378 attempts to address some of the requirements of providing a highly porous biocompatible and bioerodible device using a method of forming membranes from a polymer and particle solution. The pores are created by removing the particles, achieved by dissolving and leaching them away in a solvent, such as water, which does not dissolve the polymer, thereby leaving a porous membrane. The polymer must be soluble in a non-aqueous solvent and is limited to synthetic polymers. Once the membrane is created it may be cast into the desired shape. It is envisioned that such membranes could also be laminated together to form a three-dimensional shape.
It has been further recognized that not only the morphology of such devices but the materials of which they are composed will contribute to the regeneration processes as well as the mechanical strength of the device. For example, some materials are osteogenic and stimulate the growth of bone forming cells; some materials are osteoconductive, encouraging bone-forming cell migration and incorporation; and some are osteoinductive, inducing the differentiation of mesenchymal stem cells into osteoblasts. Materials which have been found to be osteogenic usually contain a natural or synthetic source of calcium phosphate. Osteoinductive materials include molecules derived from members of the transforming growth factor-beta (TGF-beta) gene superfamily including: bone morphogenetic proteins (BMPs) and insulin-like growth factors (IGFs).
U.S. Pat. No. 5,626,861 teaches a composite material for use as bone graft or implant composed of biodegradable, biocompatible polymer and a particulate calcium phosphate, hydroxyapatite. The calcium phosphate ceramic was added in order to increase the mechanical strength over the polymer alone and to provide a xe2x80x9cbone bondingxe2x80x9d material. The material is produced in such a manner as to provide irregular pores between 100 and 250 microns in size.
The devices described in the above-referenced U.S. patents require multiple components to be made and either placed separately in the body or pre-assembled, resulting in a complicated manipulation at the time of implant in the first case or the danger that the juncture between device components will separate post-implantation in the others.
Furthermore, these device lack a macroarchitecture or overall design that allows for the diffusion of oxygen, nutrients, and growth factors, in and out of the area in addition to a microarchitecture which creates a microenvironment which enhances cell growth and tissue regeneration.
It is therefore an object of the present invention to overcome these shortcomings, by providing a device for seeding and culturing of cells within defined regions of the device, with a pore size and porosity promoting selecting cell attachment and proliferation.
It is a further object of the present invention to provide devices which can provide mechanical support and integrity after implantation.
It is a still further object of the present invention to provide such devices which are completely biodegradable.
The devices disclosed herein are composite implantable devices having a gradient of one or more of the following: materials, macroarchitecture, microarchitecture, or mechanical properties, which can be used to select or promote attachment of specific cell types on and in the devices prior to and/or after implantation. In various embodiments, the gradient forms a transition zone in the device from a region composed of materials or having properties best suited for one type of tissue to a region composed of materials or having properties suited for a different type of tissue.
The devices are made in a continuous process that imparts structural integrity as well as a unique gradient of materials in the architecture. The gradient may relate to the materials, the macroarchitecture, the microarchitecture, the mechanical properties of the device, or several of these together. The devices disclosed herein typically are made using solid free form processes, especially three-dimensional printing process (3DP(trademark)). Other types of solid free-form fabrication (SFF) methods include stereo-lithography (SLA), selective laser sintering (SLS), ballistic particle manufacturing (BPM), and fusion deposition modeling (FDM). The device can be manufactured in a single continuous process such that the transition from one form of tissue regeneration scaffold and the other form of tissue regeneration scaffold have no xe2x80x9cseamsxe2x80x9d and are not subject to differential swelling along an axis once the device is implanted into physiological fluid.
In one embodiment for repair or replacement of bone, a gradient is formed of osteogenic and osteoconductive materials, such as calcium phosphates, to materials which are synthetic biocompatible polymers, such as poly(alpha)esters, which are particularly well suited for attachment of cells and controlled biodegradation. In another embodiment, the devices have a gradient in macroarchitecture. The macroarchitecture, or overall shape, can be of a design which allows fluid flow through and/or around one region and a different shape in another region with a gradient from one shape to the other. In another embodiment, the microarchitecture may be from an osteoinductive system of interconnected pores to a system of staggered channels inductive to chondrocyte colonization. In another aspect, the gradient may relate to mechanical properties such as tensile or compressive strength. The gradient of properties may be from that which is suitable for weight bearing loads to one which is suitable for soft tissue regeneration.
In another embodiment, materials such as growth factors, which selectively encourage or enhance the growth or differentiation of cells that form tissues, can be incorporated on or in the device. A particularly favored method of fabricating the devices includes incorporating the factors in the structure of the device.